Central Supermassive Black Holes in Seyfert Galaxies

AST 244/444 projects, Spring 2020 Seyfert galaxies

Central supermassive black holes in Seyfert galaxies

In the early 1940s, apparently following up on a suggestion by Milt Humason, Carl Seyfert discovered the class of galaxies with active nuclei that bears his name. The hallmark of a Seyfert galaxy is a relatively bright, starlike central object, easily seen in short exposures, around which long exposures reveal the bulge and disk of a spiral galaxy. Such objects are not present in the vast majority (~99%) of spiral galaxies. The central object’s luminosity is generally within a factor of ten as large as that of the rest of the galaxy, in some cases reaching a large fraction of the galaxy’s total luminosity. It is usually very difficult to reconcile the size and luminosity of the central object with the properties of normal stellar clusters. The central object also contains ionized gas for which the emission-line spectrum indicates an ionization state substantially higher than is typical of the H II regions associated with star formation. Profiles of forbidden lines of the ions of “metals” in Seyfert nuclei – by which the ionization state is analyzed – are generally relatively narrow (Doppler velocities of a few hundred km/s) and consistent with broadening primarily, but not completely, by the rotation of the galaxy. In Seyferts of type 2 this profile also characterizes the hydrogen recombination lines like Hα and Hβ. In Seyferts of type 1, however, the hydrogen recombination lines have in addition a very broad component to their profile that indicates Doppler velocities of a few thousand km/s with respect to the rest of the galaxy, much greater than shown by stellar absorption lines which characterize galactic rotation. Soon after the discovery of the nature of quasars, it was realized that quasars share many important features of Seyfert nuclei. This led to early predictions that quasars would also turn out to be galaxy nuclei – a prediction borne out shortly after the first CCD cameras enabled precise enough subtraction of the central object to reveal the surrounding galaxy. It also led the astronomical community to regard Seyfert’s work as the retrospective discovery of black holes.

Nowadays we think that the same basic supermassive-black-hole-plus-accretion-disk geometry applies to both main types of Seyferts, and that the difference of type is an artifact of the angle at which the disk is viewed: we see deeper toward the center of the disk for type 1 than type 2. This suggests that we should also see Seyfert galaxies intermediate in characteristics between these two basic types, and that has turned out to be the case, spawning Seyfert classifications like 1.5 or 1.8, in the same spirit as intermediate Hubble morphological classification (e.g. Sbc or S0/Sa).

In this project you will repeat some of Seyfert’s classic imaging1 observations, using a CCD camera instead of photographic plates, and demonstrate that Seyfert galaxies each have a starlike, extremely luminous central object which is lacking in most spiral galaxies of the same Hubble type. From this you can infer some bounds on the properties of the central black holes.

In this project, you will:

• select a Type 1 Seyfert galaxy, and another galaxy of the same morphological (Hubble) type that is not a Seyfert, taking care that the two are similar in angular size, distance, and inclination. Table 1, contains a good list of type 1 Seyferts. The normal galaxy can be selected from the online Revised Shapley-Ames catalogue. • take both short- and long-exposure LRGB images of each galaxy, taking careful note of the seeing in each frame by the sizes of stellar images, and making sure that each galaxy’s disk and spiral arms are imaged at high signal-to-noise, meaning a magnitude limit of 22-23 for RGB. • take long-exposure Hα images of each galaxy, making sure that the nuclei and brighter star- formation regions show up well. • select and average the images with the very best seeing; from this, prepare color images and one- dimensional plots of G magnitude and B-R color index in each galaxy’s nucleus, as functions of

1 Most of Seyfert’s 1942 paper is about his spectra of these galaxies, which we will not attempt to reproduce, at least not this semester.

distance from the center. Do the same for some of the brighter star-formation regions, highlighted by Hα. • average the data in each filter, omitting frames with tracking errors or unusually bad seeing • measure and account for the magnitude limit. • carefully edit the data for bad stellar images, deconvolve the L images, and prepare composite L(R+Hα)GB images of each galaxy that emphasize the nuclear differences as well as any differences in the disks.

In your analysis, answer the following questions:

• Do you reach the magnitude limit you expect? If not, why not? • Compare the brightness and color of the two galaxies’ central bulge. What differences are there? From the colors, are these old or young stellar populations? • If all has gone well, a bright unresolved object dominates the center of the Seyfert 1, which has no counterpart in the normal galaxy. What is the luminosity of this object, and how does it compare to the luminosity of the whole galaxy? What’s the maximum diameter of region in the center of the Seyfert nucleus, from which the object’s luminosity emerges? • How does the color of the Seyfert I central object compare to the colors of the large star-formation regions and young massive star clusters in both galaxies? • Can you reproduce the color and luminosity of the central object by summing up emission from stars with a plausible stellar-mass function? If so, what is the stellar density in your hypothetical central cluster, and how frequently do the stars collide? • Can you reproduce the color and luminosity of the central object with an accretion disk around a black hole? If so, what are the properties of disk and black hole? • In particular: what would be range of black-hole mass required? • Can one or other of the models be ruled out from these observations?

Include with your report your best composite L(R+Hα)GB images of your galaxies. Archive this image, and all of your raw and reduced data, in the AST 244/444 Wiki.

Additional reading

Astronomy 142, lectures 17, 18, 19, 20, 21, 22.

Martin Rees 1984, ARA&A 22, 471.

Table 1: properties of bright northern Seyfert galaxies. Adapted from Lipovetsky et al. 1988. Name J2000 coordinates Redshift Hubble Seyfert B Aperture Within aperture: α δ z0 type type (total) (arcmin) V U-B NGC 788 02 01 06.3 -06 48 50 0.0136 S0 2 13 12.76 0.48 NGC 863 02 14 33.1 -00 46 07 0.0277 Sa 1.5 13.76 1.25 12.96 -0.36 NGC 931 02 28 14.4 +31 18 47 0.0164 Sbc 1 13.67 2.29 12.78 0.34 NGC 1068 02 42 40.8 -00 00 48 0.0036 (R)Sb(r) 2 9.17 4.8 8.98 0.08 NGC 1275 03 19 48.2 +41 30 42 0.0178 S0 pec 1 12.35 2.29 11.62 0.04 NGC 1667 04 48 36.9 -06 19 12 0.0150 SBc 2 12.75 12.86 0.13 Mrk 1095 05 16 11.3 -00 08 59 0.0327 S0/Sa 1 13.6 1.25 12.87 -0.62 NGC 2110 05 52 11.4 -07 27 23 0.0077 SBa(r) 2 13.77 1.25 12.56 0.27 Mrk 3 06 15 36.0 +71 02 04 0.0141 SB0 2 13.75 1.36 12.66 0.08 NGC 2273 06 50 08.5 +60 50 45 0.0069 SB0/SBa 2 12.5 2.29 11.92 0.32 NGC 2639 08 43 38.0 +50 12 20 0.0112 (R)Sa(r) 1 12.65 1.2 11.88 0.39 NGC 2691 08 54 46.3 +39 32 13 0.0139 SB0/Sba 1 13.76 2.29 12.88 0.26 NGC 2782 09 14 05.5 +40 06 52 0.0084 Sa pec 1 12.15 1.9 11.77 -0.02 NGC 2992 09 45 41.7 -14 19 40 0.0073 Sa pec 1.9 13.17 2.29 12.04 0.51 NGC 3185 10 17 39.4 +21 41 17 0.0038 SBa(r) 1 13.23 12.15 NGC 3227 10 23 30.6 +19 51 54 0.0033 Sa(r) 1.2 11.55 12.19 0.26 NGC 3516 11 06 47.5 +72 34 07 0.0088 (R)SB0/Sba 1 12.45 12.09 0.05 NGC 3660 11 23 32.6 -08 39 45 0.0115 SBbc(r) 2 12.5 11.90 NGC 3718 11 32 37.3 +53 04 08 0.0036 SBa pec 1 11.26 6.61 10.61 NGC 3786 11 39 42.4 +31 54 35 0.0092 Sa pec 1.8 13.85 2.29 12.49 0.24 NGC 3884 11 46 13.4 +20 23 06 0.0288 Sa(r) 1 13.79 1.38 12.88 0.4 NGC 4051 12 03 09.6 +44 31 53 0.0023 Sbc(r) 1 10.95 6.92 10.18 0.02 NGC 4151 12 10 32.6 +39 24 21 0.0033 (R)Sb 1 11.13 1.07 11.15 -0.47 NGC 4235 12 17 09.9 +07 11 29 0.0077 Sa(s) 1 12.64 3.39 11.61 0.41 NGC 4253 12 18 26.7 +29 48 47 0.0128 (R)SB0/SBa 1 13.43 1.25 12.64 0.07 NGC 4258 12 18 57.5 +47 18 14 0.0020 Sbc(r) 1 8.95 16.6 10.88 0.23 NGC 4388 12 25 47.1 +12 39 42 0.0082 SBb pec 2 11.83 3.98 11.2 0.15 NGC 4448 12 28 15.0 +28 37 13 0.0023 S0/Sa 2 12 4.07 11.1 0.42 NGC 4593 12 39 39.4 -05 20 39 0.0085 (R)SBb 1 11.87 2.29 11.56 -0.02 NGC 4594 12 39 59.3 -11 37 23 0.0020 Sa(s) 1 9.27 9.25 0.64 NGC 4639 12 42 52.5 +13 15 28 0.0030 Sb 1 12.2 12.21 NGC 4941 13 04 12.5 -05 33 05 0.0029 (R)Sab 2 11.9 11.9 NGC 4939 13 04 17.7 -10 20 29 0.0098 Sbc(s) 2 12.02 4.37 11.35 0.16 NGC 5005 13 10 55.9 +37 03 32 0.0036 Sbc(r) 2 10.64 6.46 9.87 0.35 NGC 5033 13 13 26.7 +36 35 55 0.0032 Sc 1 10.6 9.12 10.16 0.27 NGC 5273 13 42 08.3 +35 39 16 0.0034 S0(s) 1 12.43 3.39 11.65 0.36 NGC 5347 13 53 16.9 +33 29 28 0.0079 SBab 2 13.46 2.24 12.7 0.04 NGC 5363 13 56 07.0 +05 15 19 0.0038 pec 2 11.2 5.5 10.25 0.62 NGC 5427 14 03 25.9 -06 01 50 0.0083 Sc pec 2 12.05 2.4 11.33 -0.06 NGC 5506 14 13 14.8 -03 12 27 0.0059 SB0/Sba 2 13.29 2.29 12.54 0.14 NGC 5548 14 17 59.5 +25 08 12 0.0168 (R)S0/Sa 1.5 13.21 2.29 12.51 -0.42 NGC 5695 14 37 23.0 +36 34 15 0.0144 SBb 2 13.55 1.25 12.71 0.25 NGC 5953 15 34 32.3 +15 11 42 0.0075 Sa pec 2 13.1 1.48 12.35 0.21 NGC 6217 16 32 29.2 +78 11 52 0.0046 (R)Sbc 2 14 11.22 -0.17 NGC 6814 19 42 40.6 -10 19 24 0.0053 Sbc 1 12.02 2.29 11.54 0.28 NGC 7469 23 03 15.6 +08 52 26 0.0167 (R)SBa 1 12.6 1.25 12.42 -0.39 NGC 7674 23 27 57.1 +08 46 49 0.0295 Sbc(r) 2 13.56 1.25 12.98 0.08 NGC 7743 23 44 21.5 +09 55 58 0.0067 (R)E/S0 2 12.42 2.95 11.43 0.41